1. Technical Field
The present invention relates generally to methods and devices for chilling food products, and more particularly to a method and apparatus for rapidly chilling liquid or slurried food products through a two-stage sequence of jacket cooling followed by vacuum chilling.
2. Background Art
Food products must be cooled after cooking to prevent pathogens from getting into the product and propagating. It is generally accepted that to keep food products safe for human consumption they must be chilled from the cooking temperature to or below 5° C. in no more than 90 minutes. This rapid cooling of food products poses a major challenge for the food processor, particularly when minimal destruction of the food product is desired.
Jacket cooling is an effective method of cooling food products if the temperature difference between the liquid and the food product is sufficient to create rapid heat transfer. This method of cooling works well down to a temperature approximately 35° C. above the temperature of the chilling liquid. If the chilling liquid is water at 5° C., the food product can be chilled down to about 40° C. efficiently. However, below 40° C. the cooling rate of the food product slows to the point that it is no longer commercially economical or practical.
FIG. 1 show the graph of the cooling curve for a thick onion and cheese sauce cooled in a jacketed vessel with 5° C. chilled water circulated through the jacket. The cooling rate is rapid for the initial thirty minutes and then levels off as the temperature difference between the food product and the chilled water is reduced. At about 40° C. the cooling rate is reduced to about 1° C. per minute, which is a marginal cooling rate in industrial food plants. If jacket cooling is continued, the cooling rate diminishes to the point that very little heat is transferred from the product to the coolant. Thus, it is impractical and inefficient to cool most food products from cooking temperature to 5° C. in 90 minutes.
Vacuum cooling is rapid at all temperatures. FIG. 2 shows the cooling curve for vacuum cooling of the same sauce as that measured in FIG. 1. The cooling rate is efficient at all temperatures and drops to 1° C. per minute rate only at a point quite close to the 5° C. temperature.
However, exclusive vacuum cooling is not without problems. At high temperatures vacuum cooling causes the product to boil violently. This violent boiling damages the food particulates in the product and splatters the inside surfaces of the vessel with sticky product. At high temperatures the product may be so unstable that if the vacuum is not precisely controlled, the product will explode into the vacuum vapor duct allowing significant amounts of product to be sucked out of the vessel and into the vacuum vapor duct. This reduces product yield and can distribute the product on the inside surfaces of the vacuum vapor duct all the way into the vacuum pump.
Controlling the vacuum process to reduce or eliminate the violent boiling is complicated. The only way to control the boiling is to slow the vacuum cooling process by reducing the vacuum level. However, any reduction of the vacuum level also slows the cooling process. Controlling the vacuum level without slowing the cooling process below a practical level is nearly impossible. An additional problem with vacuum cooling is that water moisture is evaporated out of the product during the vacuum cooling process. As the water is evaporated, flavor volatiles are also evaporated out thus reducing the flavor of the product. Jacket cooling does not take flavor volatiles out of the product.
It would be desirable, therefore, to combine jacket cooling and vacuum cooling in a method and apparatus to exploit the advantages and minimize the disadvantages of each method. By extracting most of the heat from the product with jacket cooling and using vacuum cooling for only the final stage of the chilling process, chilling efficiency is improved and fewer flavor volatiles are removed. Since the product is more stable at temperatures below the transition zone fewer flavor volatiles evaporate out at the lower temperatures.